1. Field of the Invention
The present invention relates to a digital circuit having a scan test circuit and, in particular, to a semiconductor device incorporating the scan test circuit.
2. Description of the Related Art
A scan-path testing method is known as a test facilitating technique for a semiconductor device. Flipflops in a sequential circuit in a semiconductor device are arranged to be a shift register that can be scanned (a shift register formed of a plurality of flipflops is referred to as a scan chain). In the scan-path testing method, test data is fed to an internal circuit (a combination circuit) by the flipflops while data is shifted through the shift register, and then, the shift register captures data from the internal circuit and shifts and outputs the data. In this way, the test is facilitated.
FIG. 8 shows a portion of a scan test circuit incorporated into a conventional semiconductor device.
A scan test circuit 1 shown in FIG. 8 includes selectors 12_1 through 12_4, and flipflops 10_1 through 10_4. Although scan test circuits typically include a number of selectors and flipflops, the scan test circuit 1 here includes four selectors and four flipflops for simplicity of explanation.
FIG. 8 also shows an internal circuit (a combination circuit) 11 which is tested by the scan test circuit 1.
Each of the flipflops 10_1 through 10_4 in the scan test circuit 1 has a clock input terminal K, a data input terminal D, and a data output terminal Q.
Each of the selectors 12_1 through 12_4 in the scan test circuit 1 has two data input terminals xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d, a data output terminal, and a select terminal that receives a scan enable signal SE to be discussed later.
The terminals xe2x80x9c0xe2x80x9d of the selectors 12_1 and 12_2 are connected to the internal circuit (not shown). The terminal xe2x80x9c1xe2x80x9d of the selector 12_1 is connected to an SIN (Serial In) terminal. The terminal xe2x80x9c1xe2x80x9d of the selector 12_2 is connected to the data output terminal Q of the flipflop 10_1.
The terminals xe2x80x9c0xe2x80x9d of the selectors 12_3 and 12_4 are connected to the internal circuit 11. The output terminals of the selectors 12_1 through 12_4 are respectively connected to the data input terminals D of the flipflops 10_1 through 10_4. Each clock input terminal K of the flipflops receives a clock CLK from outside the scan test circuit 1.
The data output terminals Q of the flipflops 10_1 and 10_2 are respectively connected to the internal circuit 11. The data output terminal Q of the flipflop 10_1 is also connected to the terminal xe2x80x9c1xe2x80x9d, of the selector 12_2. The data output terminal of the flipflop 10_2 is also connected to the terminal xe2x80x9c1xe2x80x9d of the selector 12_3. The data output terminal Q of the flipflop 10_3 is connected to the terminal xe2x80x9c1xe2x80x9d of the selector 12_4 and the internal circuit (not shown). The data output terminal Q of the flipflop 10_4 is connected to an SOUT (Serial Out) terminal discussed later and the internal circuit (not shown).
The scan test circuit 1 is set to be in a shift mode when the SE signal input to the selectors 12_1 through 12_4 remains at a high level. The scan test circuit 1 is set to be in a capture mode when the SE signal remains at a low level.
FIG. 9 is a timing diagram of the clock CLK and the SE signal input to the scan test circuit 1 shown in FIG. 8.
FIG. 9 illustrates the SE signal that is transitioned from a high level to a low level, and from a low level to a high level, and clock pulses T1 through T5 of the clock CLK in the order of occurrence.
With the SE signal at a high level as illustrated in FIG. 9, clock pulses T1 and T2 are generated, and with the SE signal at a low level, clock pulse T3 is generated. With the SE signal returning to a high level, clock pulses T4 and T5 are generated.
The procedure of the scan test using the scan test circuit 1 will now be explained. Referring to FIG. 9, in a state (the shift mode) during which a high-level SE signal is being input to the selectors 12_1 through 12_4 in the scan test circuit 1, test data is sent to the flipflops 10_1 through 10_4 through the selectors 12_1 through 12_4 at the timing of the rising edge of each of the clock pulses T1 and T2. The data that is sent to the flipflops 10_1 and 10_2 is also sent to the internal circuit 11.
In a state (the capture mode) during which a low-level SE signal is being input to the selectors 12_1 through 12_4 in the scan test circuit 1, the flipflops 10_3 and 10_4 capture the data, which has passed through the internal circuit 11, at the timing of the rising edge of the clock pulse T3 shown in FIG. 9.
In a state during which a high-level SE signal is being input to the selectors 12_1 through 12_4 again, the data captured by the flipflops 10_3 and 10_4 from the internal circuit 11 is successively shifted at the timing of the rising edge of each of the clock pulses T4 and T5 illustrated in FIG. 9.
The data shifted out from the flipflop 10_4 is sent out through the SOUT (Serial Out) terminal illustrated in FIG. 8. The data shifted out and the expected data are compared with each other to see if the internal circuit 11 operates normally.
The scan-path testing is thus performed on the internal circuit 11 as an object to be tested.
In step with high-speed operation, fine-line design and multi-layer wiring structure of current semiconductor devices, the probability of occurrence of signal delays due to high resistance arising from contact failure of contact holes (via holes) increases.
There is a pressing need for the scan-path testing in the high-speed operation environment to detect a signal delay failure taking place in an internal circuit.
To perform a scan-path test in a semiconductor device at a high speed in a high-speed operation environment, an expensive high-speed tester is required. This increases test costs.
Accordingly, it is an object of the present invention to provide a semiconductor device and a digital circuit, each including a scan test circuit which can perform a scan-path test at an operational environment higher in speed than the currently available environment while involving less test costs.
It is another object of the present invention to provide a method of testing a semiconductor device and a digital circuit with the semiconductor device and the digital circuit operated at a speed higher than that of a clock input from outside, by using a tester which generates a clock lower than the operational speed of the semiconductor device and the digital circuit.
To achieve the above objects, each of a semiconductor device and a digital circuit in one aspect of the present invention includes a scan chain including a plurality of pairs of a selector and a flipflop arranged in cascade, the scan chain performing a capture mode in which in response to a scan enable signal the flipflop captures data from an internal circuit in synchronization with a predetermined clock and a shift mode in which in response to the scan enable signal, one of the data stored in the flipflop and test data input from outside is shifted to a subsequent flipflop in synchronization with the predetermined clock, a clock generator which generates a clock signal in response to one of the rising edge and the falling edge of the clock pulse of another clock signal input from outside as a trigger, and a clock selector which receives the clock signal input from outside and the clock signal generated by the clock generator, and selects one of the two clock signals in response to the scan enable signal for switching between the shift mode and the capture mode, and then feeds the selected clock signal to the clock input terminal of at least one flipflop forming the scan chain.
Preferably, the clock generator generates a clock having one of the falling edge and the rising edge thereof coinciding with one of the rising edge and the falling edge of the clock signal input from outside.
Preferably, the clock generator generates a clock having one of the falling edge and the rising edge thereof delayed by a predetermined time from the timing of one of the rising edge and the falling edge of the clock signal input from outside.
A method of testing each of the semiconductor device and the digital circuit in another aspect of the present invention includes the steps of inputting the clock signal input from outside to the semiconductor device using a tester, and switching between the shift mode and the capture mode in response to the scan enable signal to test each of the semiconductor device and the digital circuit.
In each of the semiconductor device and the digital circuit of the present invention, the time interval from the last clock pulse of the clock signal selected in the shift mode to the first clock pulse of the clock signal selected in the capture mode when the shift mode is switched to the capture mode is set to be shorter than the pulse interval between adjacent pulses of the clock signal selected during the shift mode. The duration of time from the sending of data to an internal circuit to the capturing of data that has passed through the internal circuit is shorter than the time interval of the clock pulse supplied from a tester in use. A delay failure occurring in the internal circuit is thus detected using a low-cost tester when the internal circuit is operated at a speed higher than the highest frequency clock of the tester.